Add 'UserAudit'

[ardalis]

UserAudit describes how an entity was modified by a user at a particular time. The VO doesn't care about the action, just the user and the time. Thus, it should have two properties: By and On.

Example:

public class SomeEntity
{
public UserAudit Created {get; private set; }
}

Usage:
string createdByUserName = someEntity.Created.By;
DateTime createdAtDateTime = someEntity.Created.At;

Generics:
Make a base generic version with type params for both TUserIdentifier and TDateType.

Concrete versions can be created by end users. They might include user types of string, Guid, int and DateTypes of DateTime, DateTimeOffset, or NodaTime types.